'I hear a film has been made of your book! Have you seen it? Did you like it? Did you have anything to do with making it? Did you meet Colin Firth? What are you wearing to the premiere? Do you get to go to the Oscars if it's nominated? Will your other books be made into films?'

Over the past few months I've been bombarded with such questions about the upcoming film adaptation of my novel Girl with a Pearl Earring. Not just from family and friends, but from readers and journalists as well, all of them giddy with the kind of excitement I never hear over the publication of a book. No one has ever asked me what I'm going to wear to a book launch. The film world has a glamour that book publishing can never compete with - a bigger budget, flashier ads ('There's a huge billboard of your film on Sunset Boulevard!' an LA friend emailed), and glitzier awards ceremonies.

Despite this, the bright stage lights of a film don't necessarily cancel out the small, steady candle flame of a book. Indeed, that flame may last longer. These days a film has to do well in the first weekend's takings or it gets pulled from cinemas, whereas a book has a few months to earn its shelf space in a bookshop, as well as a second chance on paperback publication.

I didn't write Girl expecting it to be made into a film. Nor did I expect it to sell two million copies around the world. Of course I secretly hoped both things would happen - who doesn't dream of success? Sometimes during low points in the writing, I kept myself going by imagining the perfect cast (a younger Alan Rickman as the painter Vermeer, and some unknown actress, plucked fresh from drama school, as his servant and muse, Griet). I was aware too that the book is very visual, as well as having a straightforward story line, and so might translate easily into a film.

I researched and wrote the book in just eight months (I was pregnant and had a biological deadline). The film had a rather longer gestation. I sat in the London office of Archer Street Films, drinking cappuccino and chatting to producers Andy Paterson and Anand Tucker about selling the film option in the summer of 1999, just before the book was published. In August 2003 I first saw the finished film. Four years from first meeting to first screening is a long time in book publishing terms. I've published two novels since Archer Street bought the rights to Girl.

When I first talked with Andy and Anand, I didn't have great expectations. I was delighted they were interested in making the book into a film, but I had also heard the statistic that only 5 per cent of books optioned for films ever get made. I suspected Girl would sit on a shelf gathering dust in some film executive's office.

On the other hand, Andy and Anand were persuasive and appeared to have integrity. They seemed to 'get' the novel, understanding that the key to the story was its restraint. One of the first things I said to them was that I did not want the main characters to sleep together. Hollywood would want to sex up the film, and I thought a small British production company would be more likely to remain faithful to the book in that respect, as well as to ensure the film had a European feel to it. When they promised to try to replicate the 'emotional truth' of the book, I believed them. I was aware that I might be hoodwinked, but at least they were talking a language I would never hear in Hollywood. Does Hollywood know the meaning of restraint?

When we began negotiations, Archer Street Films asked if I wanted any involvement in writing the screenplay. I answered blithely that I wouldn't mind taking a crack at the first draft. At that point my agent Jonny Geller dragged me away from the negotiating table. 'Unless you want to become a screenwriter, don't do it!' he said. 'You're a novelist, that's what you do best. Go and write books and leave film-making to the film-makers.' It was the best advice he ever gave me. I signed off on any involvement in the film - much to the relief of Archer Street Films, I suspect. Who wants an interfering author breathing down your neck? As a result I probably had the best relationship possible with both producer Andy Paterson and screenwriter Olivia Hetreed. It also gave me an excuse in case the film turned out to be terrible - I could truthfully say that I had nothing to do with it.

Keeping the film at an arm's length meant that I didn't have to follow every twist and turn of its tortuous progress to the screen. Andy kept me informed, but usually a few months late so that by the time I found out the bad news, some good news had already come along to replace it. Bad news like: financiers pulling out, actresses 'falling out of love with the script', half-built sets being pulled down, directors likening Vermeer to an Essex man, and Hollywood starlets wanting new endings.

Instead Andy emphasised the positive: Olivia's marvellous script, actors who don't normally audition for parts queueing up to do so, Hollywood sniffing around hungrily but not allowed in. One day he told me they'd found their Griet at last. Scarlett Johansson was only 17 and not yet a household name, but experienced enough to handle the difficult role. And she resembled the painting. Then Colin Firth came on board, and it all quickly came together.

Though it had seemed like a long haul, suddenly I was on a plane to visit the set in Luxembourg. Luxembourg? That was one of many surprises about film-making. I'd thought of Delft, the Dutch town where the book is set, as very filmworthy. It still retains its seventeenth cen tury layout, with cobbled streets, canals and bridges, gables and a market square with an eight-pointed star in the centre. If you ignore all the cars, bicycles and signs, it looks like a perfect set for a film.

But no. There was a pre-built set in Luxembourg which had been used for a film set in Venice, and with a change of window shape and brick replacing Venetian stone it looked quite Dutch. (If you look closely in the film you'll see some brick-filled windows with strangely Venetian Gothic arches at the top.) I found this choice hard to fathom until I spent a few days on the set and saw just how many factors are involved in getting the look of the film right. Director Peter Webber and cinematographer Eduardo Serra needed absolute control of the space and light they worked with - something they could never achieve by shutting down a busy Delft street for an hour or two.

I had never been on a film set before. It was a surreal experience. All of the private ideas I'd had about the settings of my book were suddenly, brutally public, with cast and crew crawling all over them, measuring and moving, pulling and prodding. I kept wondering if eventually they would pull so hard the story and characters would fall apart and I'd be caught out as a fraud.

It was strange too to see that my scribbling had spawned a whole industry of Vermeer boffins - there were reproductions of Vermeers and other Dutch paintings tacked up everywhere in the production offices, and books strewn about that I myself had read for research. I confess it was rather gratifying to have Colin Firth elbow someone out of the way in the set canteen so that he could sit next to me at lunch and grill me about Vermeer. Someone else also turned to me in the canteen and said, 'Do you realise that your idea is paying all these people's wages for two months?' Yes, surreal it was.

When I first walked onto the set of Vermeer's house I immediately thought, 'No, no, this is far too big - they would never have had this kind of space.' However, once I saw just how much equipment and people were needed to film each shot, I understood what the space was for. You hardly see the dimensions of the rooms on the screen.

Moreover, all of the opulent details - the paintings hanging everywhere, the dishes, the furniture, the food - blend into the background. When watching the film we focus on Scarlett Johansson's face, not the authentic blue-and-white delftware stacked on the table that some assistant worked so hard to locate. Production designer Ben van Os had designed a spectacular room full of pornography for the house of Vermeer's patron van Ruijven, played by Tom Wilkinson; yet it doesn't even get a look-in on screen, but ended up on the cutting-room floor.

That was another surprise: how many changes were made in the editing. Much more was filmed than got used, and this is where readers of the book may most feel the differences between film and book. Whole subplots and characters were shed, and with them some of the subtleties of characterisation and ambiguities in relationships. What it gains, however, is a focused, driven plot and a sumptuous visual feast.

The changes have not bothered me. For one thing, the grammar of a film is very different from a book. By and large, films have simple story lines and are linear and active. Books - even simple, spare ones like Girl - shuttle back and forth in time, repeat themselves, go in and out of their characters' heads, and leave gaps for the reader to fill in. No wonder the storytelling has to be different. I think many films are actually less successful if they do follow a book faithfully - witness the stodginess of the Harry Potter films versus Rowling's sprightlier books.

Besides, I'm used to other people transposing what I write. After all, that is what a writer does. I wrote Girl with a Pearl Earring for readers to interpret and make their own. When I published it I also let it go. I can't control what readers think, or how they picture scenes and characters; nor do I want to. It's not so different to have film-makers do that to books. And they too have to let their vision go and let us viewers make it our own.

It would be sad if films subsumed books - and often they do. In this case, however, I think the film and book reach the same emotional truth, as Andy and Anand promised, but along slightly different paths. They are like sisters rather than the same person, and complement each other rather than fight. I now have two Griets in my head - Scarlett Johansson's luminous Griet and my own, original Griet, small and quiet - but with a steady presence no less diminished for having gained a sister.

Lust for Life (1956) Probably still the best cinematic portrait of an artist - Kirk Douglas dominates the screen as a fiery, insecure Van Gogh, while Anthony Quinn's Gauguin won him an Oscar. More restrained (and far less popular) than Vincente Minnelli's melodramatic epic is Robert Altman's Vincent & Theo (1990), studying the impact Van Gogh (Tim Roth)'s illness has on his family.

The Agony and the Ecstasy (1965) Pragmatic Pope Julius II (Rex Harrison) orders a fresco for the Sistine Chapel as a cheap substitute for rebuilding it, but reckons without the obsession of artist Michelangelo (Charlton Heston). Carol (The Third Man) Reed conjures some great moments.

Caravaggio (1986)As much a film about director Derek Jarman as a study of an artist. Fans of either the painter or the director will enjoy the way Jarman's camera adapts Caravaggio's aesthetic to the screen, but it's definitely not one for the casual viewer.

Surviving Picasso (1996) Merchant Ivory and Anthony Hopkins focus on Picasso's private life (more specifically, his sex life) in this misfire, which feels more like a failed Oscar bid than a commercial proposition.

Pollock (2000) Ed Harris's directorial debut won him an Oscar nomination for his portrayal of the splashing-and-dripping wunderkind, while Marcia Gay Harden as Pollock's wife collected the Best Supporting gong. Despite such plaudits, this uneven film never truly takes off.

Frida (2002) Salma Hayek's dream project, this biopic of Mexican artist Frida Kahlo, earned her widespread plaudits for her excellent performance. Director Julie Taymor combines conventional biography with animations based on Kahlo's own works to good effect, but mixed critical reactions confirm that there's yet to be a great biopic about a modern artist.